so youre saying that by cuting a tree i can learn to chop ppls heads off
i think the skill should be implimented because there would surely be PL(Power levelers) who train on trees if trees have high def i could take my newb sword and go chop trees instead of rats annd sell the wood and fruit couldnt i 
How could you power-level by chopping down a tree?
Think of it this way.
The tree is a monster that doesn't move, has high HP, high defence, and give about as much xp as digging a piece of ore. [I think 15 xp. thats less than killing a rat]
Imagine attacking an ulbernaught that is set on full defence. It won't kill you. but it would take a long time to kill and probably dull your weapons down a lot.
The only way to 'powerlevel' using this would be to get the practice points for a weapons skill. you basically find a good monster to kill to get XP from [to get progression points to spend on training] then go attack a tree to hone your weapons skills. Imagine it to be something like target practice.
Sure, you can loot the wood and apples from a tree, but you can also loot tails, hides, and eyes from rats which would probably take a lot less time to get a sizable amount of money from.
And yeah, I think that there should be a level cap on how much you can get from training on certain animals. A master swordsman wouldn't be able to trian his skills much using rats... and he should be able to learn something from a tree. Mainly being able to hit a stationary object. After a while, he could easily kill a tree or any other stationary enemy, but would have to train with something else if he plans to fight an enemy with the ability to move.
Though stealing from trees... I suppose a person could train a little [train to carefully twist an apple from the stem so the trees limbs don't shake when you pluck the apple] but it wouldn't be quite the same as picking from a pocket. Though I suppose if a person has a hankerchief hanging out of their pocket, it would be about the same difficulty. [taking a henkercheif without the person noticing as opposed to taking an apple without the limbs moving].
But in the end, I just mentioned using pickpocket to get apples because its the in-game way to get items from a living thing without killing them. I mean, unless you '/use' a tree to get fruit from it or something... but that would make them objects instead of monsters...
Or make them like mines so you '/pick apples' for apples and if you try it on an orange tree your character says "I don't see a good place to pick fruit..."